49 SOME NOTES ON LOCAL RAINFALL. 
It will be observed that the greatest rate of rainfall in the period 
under notice has not cccurred upon days when there has been the 
greatest amount of rain, and that the period of most rapid falls is the 
six montls, April to October, the months of June, July, and August 
being those when the maximum rate of fall may be expected. 
With the limited information at disposal, it is hardly possible to 
draw any particular conclusions as to what rate of fallin a given 
period may be expected, but for practical purposes, and after making 
allowance for evaporation, absorption, and time of running off, I have 
adopted a standard of half-an-inch of rain per hour from impervious 
surfaces to be drained, and so far have found this allowance sufficient, 
although it is open to correction if future observations differ much 
from those already made. I was once connected witha town in 
Lancashire where an allowance of three times this rainfall was made, 
and where in a very severe thunderstorm, in August, 1889, I think, 
12. in. rain fell in 55 minutes. The drainage works designed to deal 
with 13 inches of rain per hour had just been completed, and proved 
sufficient for the occasion. 
I have an idea that there is some proper proportion between the 
average annual rainfall and the maximum quantity and rate in a 
given time, but for want of sufficient information [ am unable to 
formulate a theory with any degree of confidence. So far as my 
experience goes, it would appear as if the maximum hourly fall in 
different places is about proportional to the annual fal], and as if 
the maximum rate of fall is inversely proportional to the square 
root of the duration. 
Diagram No. 2 (20th July, 1900) shows a fall of 0°63 in., 0°6 in. 
of which fell in 27 minittes, and the greatest rate was 3 in. per hour, 
‘4 in. falling in 8 minutes. In this case the fall registered at the Grange 
